DNA binding of USF is required for specific E-box dependent gene activation in vivo

Although USF-1 and -2 are the major proteins that bind to Myc-regulated E-box (CACGTG) elements in many cells, there is no clear role for USF during Myc-dependent gene regulation. Using dominant negative alleles of USF-1 we now show that DNA binding by USF at a Myc-regulated E-box limits the ability of another E-box binding factor, TFE-3, to activate a target gene of Myc in vivo and to stimulate S phase entry in resting fibroblasts. Similarly, dominant negative alleles of USF-1 relieve the restriction that prevents activation of the IgH enhancer by TFE-3 in non B-cells. DNA binding activity of USF complexes is abundant in primary human B-cells and is significantly downregulated during B-cell immortalization. Re-expression of USF-1 in immortalized B-cells retards proliferation. Our data establish an essential role for USF in restricting E-box dependent gene activation in vivo and suggest that this control is relaxed during cellular immortalization.

Direct induction of cyclin D2 by Myc contributes to cell cycle progression and sequestration of p27

Ectopic expression of Myc induces Cdk2 kinase activity in quiescent cells and antagonizes association of p27(kip1) with Cdk2. The target gene(s) by which Myc mediates this effect is largely unknown. We now show that p27 is rapidly and transiently sequestered by cyclin D2-Cdk4 complexes upon activation of Myc and that cyclin D2 is a direct target gene of Myc. The cyclin D2 promoter is repressed by Mad-Max complexes and de-repressed by Myc via a single highly conserved E-box element. Addition of trichostatin A to quiescent cells mimics activation of Myc and induces cyclin D2 expression, suggesting that cyclin D2 is repressed in a histone deacetylase-dependent manner in quiescent cells. Inhibition of cyclin D2 function in established cell lines, either by ectopic expression of p16 or by antibody injection, inhibits Myc-dependent dissociation of p27 from Cdk2 and Myc-induced cell cycle entry. Primary mouse fibroblasts that are cyclin D2-deficient undergo accelerated senescence in culture and are not immortalized by Myc; induction of apoptosis by Myc is unimpaired in such cells. Our data identify a downstream effector pathway that links Myc directly to cell cycle progression.

Thrombopoietin acts synergistically on Ca(2+) mobilization in platelets caused by ADP or thrombin receptor agonist peptide

Thrombopoietin (TPO) is the main regulator of megakaryopoiesis and influences also the function of mature platelets. TPO has been shown to synergize in multiple platelet activation processes induced by various agonists. Our aim was to elucidate whether TPO affects calcium signaling during platelet activation processes. TPO demonstrated a synergistic effect on the exocytosis induced by suboptimal doses of adenosine diphosphate (ADP) and the thrombin receptor agonist peptide (TRAP). We detected synergistic effects of TPO on the ADP or TRAP induced Ca(2+) mobilization in a small range of very low agonist concentrations. The TPO synergism on Ca(2+) mobilization and CD62P expression was measurable in different, nonoverlapping ranges of ADP or TRAP concentrations. Sustaining the agonist-induced calcium signal with thapsigargin led to a detectable TPO synergism in CD62P expression even in agonist concentrations in which the synergism only occurs in Ca(2+) signaling without thapsigargin.

Helix packing in polytopic membrane proteins: role of glycine in transmembrane helix association

The nature and distribution of amino acids in the helix interfaces of four polytopic membrane proteins (cytochrome c oxidase, bacteriorhodopsin, the photosynthetic reaction center of Rhodobacter sphaeroides, and the potassium channel of Streptomyces lividans) are studied to address the role of glycine in transmembrane helix packing. In contrast to soluble proteins where glycine is a noted helix breaker, the backbone dihedral angles of glycine in transmembrane helices largely fall in the standard alpha-helical region of a Ramachandran plot. An analysis of helix packing reveals that glycine residues in the transmembrane region of these proteins are predominantly oriented toward helix-helix interfaces and have a high occurrence at helix crossing points. Moreover, packing voids are generally not formed at the position of glycine in folded protein structures. This suggests that transmembrane glycine residues mediate helix-helix interactions in polytopic membrane proteins in a fashion similar to that seen in oligomers of membrane proteins with single membrane-spanning helices. The picture that emerges is one where glycine residues serve as molecular notches for orienting multiple helices in a folded protein complex.

Bin1 functionally interacts with Myc and inhibits cell proliferation via multiple mechanisms

The tumor suppressor Bin1 was identified through its interaction with the N-terminal region of Myc which harbors its transcriptional activation domain. Here we show that Bin1 and Myc physically and functionally associate in cells and that Bin1 inhibits cell proliferation through both Myc-dependent and Myc-independent mechanisms. Bin1 specifically inhibited transactivation by Myc as assayed from artificial promoters or from the Myc target genes ornithine decarboxylase (ODC) and alpha prothymosin (pT). Inhibition of ODC but not pT required the presence of the Myc binding domain (MBD) of Bin1 suggesting two mechanisms of action. Consistent with this possibility, a non-MBD region of Bin1 was sufficient to recruit a repression function to DNA that was unrelated to histone deacetylase. Regions outside the MBD required for growth inhibition were mapped in Ras cotransformation or HepG2 hepatoma cell growth assays. Bin1 required the N-terminal BAR domain to suppress focus formation by Myc whereas the C-terminal U1 and SH3 domains were required to inhibit adenovirus E1A or mutant p53, respectively. All three domains contributed to Bin1 suppression of tumor cell growth but BAR-C was most crucial. These findings supported functional interaction between Myc and Bin1 in cells and indicated that Bin1 could inhibit malignant cell growth through multiple mechanisms.

Control of cell proliferation by Myc family genes

The c-myc proto-oncogene was discovered as the cellular homologue of the transforming oncogene of several chicken retroviruses. It has become clear that c-myc is a member of a small gene family; members of this family encode transcription factors of the Helix-loop-Helix family of transcription factors that can both activate and repress transcription. In vivo, these genes control both proliferation and apoptosis. This review will focus on recent progress in understanding how the encoded proteins exert their biological functions.

Magic angle spinning NMR of the protonated retinylidene Schiff base nitrogen in rhodopsin: expression of 15N-lysine- and 13C-glycine-labeled opsin in a stable cell line

The apoprotein corresponding to the mammalian photoreceptor rhodopsin has been expressed by using suspension cultures of HEK293S cells in defined media that contained 6-15N-lysine and 2-13C-glycine. Typical yields were 1.5-1.8 mg/liter. Incorporation of 6-15N-lysine was quantitative, whereas that of 2-13C-glycine was about 60%. The rhodopsin pigment formed by binding of 11-cis retinal was spectrally indistinguishable from native bovine rhodopsin. Magic angle spinning (MAS) NMR spectra of labeled rhodopsin were obtained after its incorporation into liposomes. The 15N resonance corresponding to the protonated retinylidene Schiff base nitrogen was observed at 156.8 ppm in the MAS spectrum of 6-15N-lysine-labeled rhodopsin. This chemical shift corresponds to an effective Schiff base-counterion distance of greater than 4 A, consistent with structural water in the binding site hydrogen bonded with the Schiff base nitrogen and the Glu-113 counterion. The present study demonstrates that structural studies of rhodopsin and other G protein-coupled receptors by using MAS NMR are feasible.

Control of cell proliferation by Myc proteins

Taken together, the available data appear to be consistent with a model in which Myc proteins function downstream of D-type cyclins and synergize with E2F proteins in the activation of the cyclin E/cdk2 kinase. This view of Myc proteins appears strikingly similar to established models for the E2F/DP family of proteins. However, it should be noted that there are clear differences and several predictions of such a model that have been critically tested for E2F proteins are still untested for Myc in this model. First, it appears that at least some target genes of Myc implicated in this process are still unknown; second, clear data from knockout cells that link p107 to Myc function are missing; and third, we are not aware of studies of tumour samples that clarify whether mutations in myc genes relieve the requirement for mutations in the cyclin D/p16 pathway.

Control of cell proliferation by Myc

Myc proteins are key regulators of mammalian cell proliferation. They are transcription factors that activate genes as part of a heterodimeric complex with the protein Max. This review summarizes recent progress in understanding how Myc stimulates cell proliferation and how this might contribute to cellular transformation and tumorigenesis.

The functions of Myc in cell cycle progression and apoptosis

c-myc has emerged as one of the central regulators of mammalian cell proliferation. The gene encodes a transcription factor of the HLH/leucine zipper family of proteins that activates transcription as part of a heteromeric complex with a protein termed Max. In mammalian fibroblasts, Myc acts as an upstream regulator of cyclin-dependent kinases and functionally antagonises the action of at least one cdk inhibitor, p27. Myc also induces cells to undergo apoptosis, and the relationship between Myc-induced cell cycle entry and apoptosis is discussed.

Cdk2-dependent phosphorylation of p27 facilitates its Myc-induced release from cyclin E/cdk2 complexes

Activation of Myc triggers a rapid induction of cyclin E/cdk2 kinase activity and degradation of p27. Overt degradation of p27 is preceded by a specific dissociation of p27 from cyclin E/cdk2, but not from cyclin D/cdk4 complexes. We now show that cyclin E/cdk2 phosphorylates p27 at a carboxy-terminal threonine residue (T187) in vitro; mutation of this residue to valine stabilises cyclin E/cdk2 complexes. This reaction is not significantly inhibited by high concentrations of p27, suggesting that cdk2 bound to p27 is catalytically active. In vivo, p27 bound to cyclins E and A, but not to D-type cyclins is phosphorylated. Myc-induced release of p27 from cdk2 requires cdk2 kinase activity and is delayed in a T187V mutant of p27. After induction of Myc, p27 phosphorylated at threonine 187 transiently accumulates in a non cdk2 bound form. Our data suggest a mechanism in which p27 is released from cyclin E/cdk2 upon phosphorylation; in Myc-transformed cells, release is efficient as phosphorylated p27 is transiently bound in a non-cdk2 containing complex and subsequently degraded.

Association of Myc with the zinc-finger protein Miz-1 defines a novel pathway for gene regulation by Myc

The Myc protein activates transcription as part of a complex with its partner protein, Max. Myc-transformed cells are also characterised by the loss of expression of a number of genes and this repressive effect of Myc on gene expression may not be mediated by the Myc/Max complex. We recently isolated by two-hybrid cloning a novel zinc-finger protein that associates with the carboxy-terminus of Myc. We have termed this protein Miz-1 (Myc-interacting zinc finger protein). Some of the properties of Miz-1 suggest that it may be involved in gene repression by Myc in vivo.

An alternative pathway for gene regulation by Myc

The c-Myc protein activates transcription as part of a heteromeric complex with Max. However, Myc-transformed cells are characterized by loss of expression of several genes, suggesting that Myc may also repress gene expression. Two-hybrid cloning identifies a novel POZ domain Zn finger protein (Miz-1; Myc-interacting Zn finger protein-1) that specifically interacts with Myc, but not with Max or USF. Miz-1 binds to start sites of the adenovirus major late and cyclin D1 promoters and activates transcription from both promoters. Miz-1 has a potent growth arrest function. Binding of Myc to Miz-1 requires the helix-loop-helix domain of Myc and a short amphipathic helix located in the carboxy-terminus of Miz-1. Expression of Myc inhibits transactivation, overcomes Miz-1-induced growth arrest and renders Miz-1 insoluble in vivo. These processes depend on Myc and Miz-1 association and on the integrity of the POZ domain of Miz-1, suggesting that Myc binding activates a latent inhibitory function of this domain. Fusion of a nuclear localization signal induces efficient nuclear transport of Miz-1 and impairs the ability of Myc to overcome transcriptional activation and growth arrest by Miz-1. Our data suggest a model for how gene repression by Myc may occur in vivo.

Activation of c-Myc uncouples DNA replication from activation of G1-cyclin-dependent kinases

Proto-oncogenes like c-myc are thought to control exit from the cell cycle rather than progression through the cell cycle itself. We now present a different view of Myc function. Exponentially growing Rat1-MycER fibroblasts were size-fractionated by centrifugal elutriation. In these cells, activation of cyclin E- and cyclin A-dependent kinases, degradation of p27, hyperphosphorylation of retinoblastoma protein and activation of E2F occur sequentially at specific cell sizes. Upon activation of Myc, however, these transitions all occur simultaneously in small cells immediately after exit from mitosis. In contrast, Myc has no discernible effect on the cell size at which DNA replication is initiated. These data show first that Myc controls the activity of G1 cyclin-dependent kinases independently from the transition between quiescence and proliferation and from any effect on cell growth in size. These data also provide evidence of at least one dominant mechanism besides activation of E2F and of cyclin E/cdk2 kinase, which prevents DNA replication unless a critical cell size has been reached.

Transcriptional control: calling in histone deacetylase

Mad proteins are transcriptional repressors that antagonize transcriptional activation and transformation by Myc oncoprotein; recent findings suggest that they repress transcription by recruiting histone deacetylases to target sites on DNA.

Mutual requirement of CDK4 and Myc in malignant transformation: evidence for cyclin D1/CDK4 and p16INK4A as upstream regulators of Myc

We demonstrate in this paper that CDK4 which is a G1 phase specific cell cycle regulator and catalytic subunit of D-type cyclins has oncogenic activity similar to D-type cyclins themselves and is able to provoke focus formation when cotransfected with activated Ha-ras into primary rat embryo fibroblasts. Surprisingly, using two different mutants we show that CDK4's ability to bind to p16INK4a and not its kinase activity is important for its transforming potential. In addition, p16INK4a but not a mutant form that is found in human tumours can completely abrogate focus formation by CDK4 suggesting that CDK4 can malignantly transform cells by sequestering p16INK4a or other CKIs. We demonstrate that both cyclin D1 and CDK4 functionally depend on active Myc to exert their potential as oncogenes and vice versa that the transforming ability of Myc requires functional cyclin D/CDK complexes. Moreover, we find that p16INK4a and the Rb related protein p107 which releases Myc after phosphorylation by cyclin D1/CDK4 efficiently block Myc's activity as a transcriptional transactivator and as an oncogene. We conclude that both p16INK4a and cyclin D/CDK4 complexes are upstream regulators of Myc and directly govern Myc function in transcriptional transactivation and transformation via the pocket protein p107.

Transactivation of prothymosin alpha and c-myc promoters by human papillomavirus type 16 E6 protein

The human papillomavirus type 16 E6 protein exerts a transforming activity through inactivation of tumor suppressor p53. Recently E6 has been shown to have additional transforming activities independent of p53. E6 is able to transactivate or repress several specific viral promoters. However, underlying molecular mechanisms and cellular target genes for the activity are not well understood. Using a differential hybridization technique, we identified the prothymosin alpha gene as a cellular target of E6 transactivation. E6 was able to transactivate the prothymosin alpha promoter in H358 cells lacking p53 and in C33A cells harboring a mutant p53 allele. Disruption of the E-box in intron 1 of the prothymosin alpha promoter abolished the responsiveness to E6. Then we determined if E6 up-regulates the expression of Myc, by which the prothymosin alpha promoter is transactivated through the E-box. We found that E6 is also able to transactivate the c-myc promoter in H358 cells and in C33A cells. These results suggest that E6 is able to transactivate the c-myc promoter independently of p53, and that the prothymosin alpha promoter is subsequently transactivated by Myc.

Myc: a single gene controls both proliferation and apoptosis in mammalian cells

c-myc was discovered as the cellular homologue of the transduced oncogene of several avian retroviruses. The gene encodes a transcription factor, which forms a heteromeric protein complex with a partner protein termed Max. In mammalian cells, Myc is a central regulator of cell proliferation and links external signals to the cell cycle machinery. Myc also induces cells to undergo apoptosis, unless specific signals provided either by cytokines or by oncogenes block the apoptotic pathway. Recent progress sheds light both on the factors regulating the function and expression of Myc and on the downstream targets in the cell cycle. Together, these findings suggest the existence of a novel signal transduction pathway regulating both apoptosis and proliferation.

Cell cycle regulation of the murine cyclin E gene depends on an E2F binding site in the promoter

Cyclin E controls progression through the G1 phase of the cell cycle in mammalian fibroblasts and potentially in many other cell types. Cyclin E is a rate-limiting activator of cdk2 kinase in late G1. The abundance of cyclin E is controlled by phase-specific fluctuations in the mRNA level; in mammalian fibroblasts, mRNA is not detected under conditions of serum starvation and is accumulated upon serum stimulation, with expression starting in mid-G1. Here, we report the cloning of the murine cyclin E promoter. We isolated a 3.8-kb genomic fragment that contains several transcriptional start sites and confers cell cycle regulation on a luciferase reporter gene. This fragment also supports transcriptional activation by adenovirus E1A, a known upstream regulator of cyclin E gene expression. An E2F binding site which is required for G1-specific activation of the cyclin E promoter in synchronized NIH 3T3 cells was identified in this fragment.

Activation of cyclin-dependent kinases by Myc mediates induction of cyclin A, but not apoptosis

The activation of conditional alleles of Myc induces both cell proliferation and apoptosis in serum-deprived RAT1 fibroblasts. Entry into S phase and apoptosis are both preceded by increased levels of cyclin E- and cyclin D1-dependent kinase activities. To assess which, if any, cellular responses to Myc depend on active cyclin-dependent kinases (cdks), we have microinjected expression plasmids encoding the cdk inhibitors p16, p21 or p27, and have used a specific inhibitor of cdk2, roscovitine. Expression of cyclin A, which starts late in G1 phase, served as a marker for cell cycle progression. Our data show that active G1 cyclin/cdk complexes are both necessary and sufficient for induction of cyclin A by Myc. In contrast, neither microinjection of cdk inhibitors nor chemical inhibition of cdk2 affected the ability of Myc to induce apoptosis in serum-starved cells. Further, in isoleucine-deprived cells, Myc induces apoptosis without altering cdk activity. We conclude that Myc acts upstream of cdks in stimulating cell proliferation and also that activation of cdks and induction of apoptosis are largely independent events that occur in response to induction of Myc.

Discrimination between different E-box-binding proteins at an endogenous target gene of c-myc

c-myc plans a key role in regulating mammalian cell proliferation and apoptosis. The gene codes for a transcription factor, Myc, that belongs to the helix-loop-helix/leucine zipper (HLH/LZ) family of proteins. Myc heterodimerizes with a partner protein termed Max; the heterodimeric complex binds to CAC(G/A)TG (E-box) sequences and activates transcription from these sites. However, several other HLH/LZ proteins, including USF and TFE-3, bind to and trans-activate from the same element, yet have no documented effect on cell proliferation or apoptosis. Therefore, it is likely that mechanisms exist that discriminate between these proteins for activation of natural target genes of Myc. We now show that trans-activation from the E-box in the rat prothymosin-alpha intron enhancer is indeed specific for Myc, and identify both the distance from the start site of transcription and a second E-box element adjacent to that recognized by Myc as critical determinants of specificity. Surprisingly, transcription activation domains required for Myc to activate from this distal enhancer position differ from previously mapped domains and closely correlate with those domains essential for transformation. As observed in transformation assays, Myc and Max strongly synergize in activation from a distal enhancer position. Our data suggest that trans-activation from the prothymosin intron enhancer is a faithful reflection of the transforming properties of the Myc protein.

Expression of cyclin D1 mRNA is not upregulated by Myc in rat fibroblasts

Conflicting results have been published regarding the regulation of cyclin D1 mRNA in rat fibroblasts expressing a hormone-regulated Myc protein, MycER. We confirm that activation of MycER with oestrogen rapidly induces cyclin D1 mRNA, even in the presence of cycloheximide. However, we show that this is an artefact resulting from an oestrogen-activated transcriptional activation domain in the oestrogen receptor part of the MycER chimaera. First, addition of 4-hydroxy-tamoxifen (4OHT), which does not activate this domain, allows association of MycER with Max and induces cell proliferation in serum-starved Rat-1-MycER cells without affecting cyclin D1 mRNA levels or the activity of D1 promoter-luciferase constructs. Second, Rat-1 cells expressing a mutant MycER with a hormone-binding domain that still binds 4OHT but no longer binds oestrogen, are driven into the cell cycle in response to 4OHT but fail to up-regulate cyclin D1 mRNA. Finally, Rat-1 cells in which wild-type human c-Myc expression can be induced, also progress into the cell cycle without increased D1 mRNA expression.

Identification of a Myc-dependent step during the formation of active G1 cyclin-cdk complexes

Activation of conditional alleles of Myc can induce proliferation in quiescent cells. We now report that induction of Myc in density-arrested fibroblasts triggers rapid hyperphosphorylation of the retinoblastoma protein and activation of both cyclin D1- and cyclin E-associated kinase activities in the absence of significant changes in the amounts of cyclin-cdk complexes. Kinase activation by Myc is blocked by inhibitors of transcription and requires intact DNA binding and heterodimerization domains of Myc. Activation of cyclin E-cdk2 kinase in serum-starved cells occurs in two steps. The first is induced by Myc and involves the release of a 120 kDa cyclin E-cdk2 complex from a 250 kDa inactive complex that is present in starved cells. This is necessary, but not sufficient, to generate full kinase activity, as cdc25 phosphatase activity is limiting in the absence of external growth factors. In vivo cdc25 activity can be supplied by the addition of growth factors. In vitro recombinant cdc25a strongly activates the 120 kDa, but only poorly activates the 250 kDa cyclin E-cdk2 complex. Our data show that two distinct signals, one of which is supplied by Myc, are necessary for consecutive steps during growth factor-induced formation of active cyclin E-cdk2 complexes in G(o)-arrested rodent fibroblasts.